The present invention relates to a method and apparatus for controlling an output current of a controlled rectifier by means of phase control.
As an example of an AC motor system with a speed variable drive system using a controlled rectifier, a brushless motor system has been known. As illustrated in FIG. 1, an example of brushless motor system comprises a controlled rectifier 2 connected to a three-phase AC power supply 1, a DC reactor for smoothing the ripple in the DC output from the controlled rectifier 2, an inverter 4 for converting the DC output from the controlled rectifier 2 into an AC output of a desired frequency. The output of the inverter 4 is fed to a three-phase synchronous motor 5 for driving the same. The angular position of the motor 5 is detected by an angular position detector 6. A pulse generator 7 produces, responsive to the output of the detector 6, gate signals for each of the thyristors of the inverter 4. A reference value I.sub.o of the rectifier output current is given by a reference generator 8. The output current I.sub.dc of the rectifier 2 is detected by a current detector 9. The reference value I.sub.o and the detected current I.sub.dc are compared with each other and a control signal is produced in accordance with the result of the comparison at the controller 10, i.e., the difference .DELTA.I=I.sub.o -I.sub.dc. The control signal from the comparator 10 is fed to a firing pulse generator 11 which applies gate signals to each of the thyristers of the controlled rectifier 2 at angles determined in accordance with the output of the controller 10. A synchronizing pulse generator 12 detects the output of the power supply 1 and supplies a synchronizing pulse P.sub.c at an interval of the average firing period to the firing pulse generator for ensuring synchronism.
The system shown in FIG. 1 has the following problems. When the frequency of the output of the motor 5 is close to the frequency of the power supply 1, a current beat of a frequency related to the difference between the power supply frequency and the inverter output frequency occurs.
More particularly, assume that the motor frequency is constant and close to the power supply frequency and the firing angle of the controlled rectifier 2 is fixed. The DC output voltage V.sub.dc1 of the controlled rectifier 2 and the input voltage V.sub.dc2 of the inverter 4 are as shown in FIGS. 2A and 2B, respectively. Accordingly, the voltage V.sub.dcL across the reactor 3 is as shown in FIG. 2C. The reactor 3 is provided to accommodate or "absorb" the voltage difference. The reactance of the reactor 3 is selected to be as small as possible for economy and for shorter response time in current control. As a result, pulsation (hereinafter referred to as current ripple) with a frequency equal to the firing frequency (six times the power supply frequency) appears in the current I.sub.dc, as illustrated in FIG. 2D, and the current ripple of the current I.sub.dc is amplitude-modulated at a frequency six times the difference between the power supply frequency and the motor frequency. The average value of the current I.sub.dc is substantially constant. Therefore, if the motor 5 is driven by an AC current corresponding to the current I.sub.dc, the average torque is constant, i.e., although pulsation corresponding to the firing frequency occurs, pulsation or variation at a lower frequency does not occur, so that there is no problem in current control. Thus, as far as the firing angle is constant there is no problem. However, when the rectifier output current is fed back to effect current control, the ripple component of the load current is included in the output of the controller 10 which is supplied to the firing pulse generator 11 as a firing angle reference and hence a different phenomenon occurs. This phenomenon is explained below.
To simplify the explanation, it is assumed that the controller 10 is of a proportional control type, the current reference is constant, and the firing angle is constant. The output .DELTA.I of the controller 10 is a difference between the constant current reference I.sub.o and the detected current I.sub.dc, so that the wave form of the output .DELTA.I is, as shown in FIG. 2E, substantially an inversion of the current I.sub.dc. The broken line in FIG. 2E indicates the instantaneous values of the output .DELTA.I at the instances when the firing pulses are given. The instantaneous values indicated by the broken line are related to the firing angle. Thus, the assumption that the firing angle is constant is revealed to be incorrect. In reality, the firing angle varies when a feedback control is carried out. The variation in the firing angle is related in magnitude to the current ripple and has a frequency six times that of the difference between the power supply frequency and the motor frequency. As a result, the average value of the rectifier output current also has a pulsation, i.e., current beat of the same frequency.
In order to prevent the effect of the load current beats on the output of the controller 10 incorporation of a low-pass filter in the control system may be conceived. However, incorporation of a low-pass filter prolongs the response time, and hence should be avoided.
Another problem is that it is difficult to make the conventional current control system of digital data processing devices. One recent trend in automatic control is to change from analog data processing to digital data processing. This is because the digital techniques such as microcomputer techniques have developed, the digital data processing can now be carried out easily and economically by the use of a microcomputer or the like, use of digital data processing reduces the number of elements or parts which require adjustment and improves reliability, and the current reference is often given in a digital form where the control system is used in a large scale system such as a steel production line and the large scale system is under control of a computer system.
If the controller and the firing angle generator are made of digital devices by, for example, the use of a microcomputer, the load current value has to be digitized before it is supplied to the current control circuit. To fully reproduce the wave form of the rectifier output current, the sampling must be made at a high frequency. Also, the program for carrying out the sampling is complicated, and substantial time is required to execute the program. These facts may lead to reduction of time for operation of comparison and firing pulse generation. Thus, it will be appreciated that there are difficulties in making the conventional current control system of digital data processing devices.